Phenol is an important product in the chemical industry with utility in, for example, the production of phenolic resins, ε-caprolactam, adipic acid, plasticizers and particularly Bisphenol A. The demand for phenol for the manufacture of Bisphenol-A and subsequently polycarbonates is accelerating, owing to the broadening applications of polycarbonates in the electronic, healthcare, and automobile industries.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is normally a three-step process involving alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone.
The rapid growth of cumene, phenol and Bisphenol-A production, however, has caused some concerns related to the imbalance of the acetone byproduct produced from the phenol plant. Thus, acetone and phenol are produced at an approximately 1:1 molar ratio from cumene, but are used at an approximately 1:2 molar ratio in the downstream Bisphenol-A production process. The excess acetone that is not used in the production of Bisphenol-A has caused some concern from phenol producers in that it may create a supply-demand imbalance and disrupt the economics of the phenol production business. In addition, the cost of propylene is likely to increase, due to a developing shortage of propylene. Thus, a process that produces cumene, with less or no propylene as a feed, may be an attractive alternative route to the production of phenol.
Numerous research efforts have been directed at solving the acetone imbalance and propylene supply issues described above by recycling the excess acetone produced in the phenol plant to produce cumene. For example, U.S. Pat. No. 5,015,786 teaches a process for preparing phenol, comprising the steps of: (a) alkylating benzene with isopropanol using a zeolite catalyst under liquid phase conditions to synthesize cumene, (b) oxidizing the cumene from step (a) with molecular oxygen into cumene hydroperoxide, (c) subjecting cumene hydroperoxide to acid cleavage to synthesize phenol and acetone, and (d) hydrogenating the acetone from step (c) with hydrogen gas under liquid phase conditions into isopropanol which is recycled to step (a).
One problem in making cumene from isopropanol produced from the excess acetone from a phenol plant is that the acetone tends to contain significant quantities of nitrogen impurities which carry over into the isopropanol intermediate product. Such impurities act as poisons to the zeolite catalyst employed in the downstream alkylation step and so must be removed or reduced to very low levels. However, attempts to remove these impurities from the acetone and isopropanol feeds with conventional solid acid adsorbents have proved to be only marginally effective due to the molecular polarity of the acetone and isopropanol, which competes with the adsorption of the polar nitrogen compounds. Also, the high water solubility of acetone and isopropanol eliminates the use of water washing, which is otherwise commonly employed to remove nitrogen compounds from hydrocarbon streams.
In accordance with the present invention, it has now been found that one important contributor to the nitrogen impurities in the acetone intermediate is the caustic washing steps that are conventionally used to prevent build-up of organic acids in the phenol plant, for example during the cumene oxidation and phenol recovery stages. In particular, it has been found that nitrogen compounds employed in the phenol plant as corrosion inhibitors are transferred into the organic cumene phase during caustic washing of various cumene recycle streams. Transfer of these nitrogen compounds into the cumene recycle streams to the oxidation step results in formation of nitrogen impurities in the acetone product. However, cumene, unlike acetone and isopropanol, is susceptible to purification by conventional solid acid treatment and water washing. Thus the present invention seeks to avoid the problem of purification of the downstream acetone and isopropanol streams, by effecting nitrogen removal from the cumene recycle streams upstream in the phenol plant.